Biomaterials have long been used to fabricate biomedical devices for use in both in vitro and in vivo applications. A variety of biomaterials can be used for the fabrication of such devices, including ceramics, metals, polymers, and combinations thereof. Historically, such biomaterials were considered suitable for use in fabricating biomedical devices if they provided a suitable combination of such basic properties as inertness, low toxicity, and the ability to be fabricated into desired devices. (Hanker, J. S. and B. L. Giammara, Science 242:885-892, 1988).
As the result of more recent advances, devices can now be provided with surfaces having various desirable characteristics, e.g., in order to better interface with surrounding tissue or solutions. For instance, approaches have been developed to promote the attachment of specific cells or molecules to device surfaces. A device surface, for instance, can be provided with a bioactive group that is capable of attracting and/or attaching to various molecules or cells. Examples of such bioactive groups include antigens for binding to antibodies, ligands for binding to cell surface receptors, and enzyme substrates for binding to enzymes.
Such bioactive groups have been provided on the surfaces of biomaterials in a variety of ways. In one approach, biomaterials can be fabricated from molecules that themselves present the desired bioactive groups on the surfaces of devices after fabrication. However, desirable bioactive groups are typically hydrophilic and cannot be incorporated into most metals or hydrophobic polymeric biomaterials at effective concentrations without disrupting the structural integrity of such biomaterials.
An alternative approach involves adding bioactive groups to the surfaces of biomaterials, e.g., after they have been fabricated into medical devices. Such bioactive groups can occasionally be added by adsorption. However, groups that have been added by adsorption cannot typically be retained on surfaces at high levels or for long periods of time.
The retention of such bioactive groups on a surface can be improved by covalent bonding of those groups to the surface. For instance, U.S. Pat. Nos. 4,722,906, 4,979,959, 4,973,493 and 5,263,992 relate to devices having biocompatible agents covalently bound via a photoreactive group and a chemical linking moiety to the biomaterial surface. U.S. Pat. Nos. 5,258,041 and 5,217,492 relate to the attachment of biomolecules to a surface through the use of long chain chemical spacers. U.S. Pat. Nos. 5,002,582 and 5,263,992 relate to the preparation and use of polymeric surfaces, wherein polymeric agents providing desirable properties are covalently bound via a photoreactive moiety to the surface. In particular, the polymers themselves exhibit the desired characteristics, and in the preferred embodiment, are substantially free of other (e.g., bioactive) groups.
Others have used photochemistry to modify the surfaces of biomedical devices, e.g., to coat vascular grafts. (See, e.g., Kito, H. et. al., ASAIO Journal 39:M506-M511, 1993. See also Clapper, D. L., et. al., Trans. Soc. Biomat. 16:42, 1993).
Cholakis and Sefton synthesized a polymer having a polyvinyl alcohol (PVA) backbone and heparin bioactive groups. The polymer was coupled to polyethylene tubing via nonlatent reactive chemistry, and the resultant surface was evaluated for thromboresistance in a series of in vitro and in vivo assays. For whatever reason, the heparin in the polymer prepared by Cholakis and Sefton did not provide effective activity. (Cholakis, C. H. and M. V. Sefton, J. Biomed. Mater. Res. 23:399-415, 1989. See also Cholakis, C. H., et. al., J. Biomed. Mater. Res. 23:417-441, 1989).
Finally, Kinoshita et. al. disclose the use of reactive chemistry to generate polyacrylic acid backbones on porous polyethylene, with collagen molecules being subsequently coupled to carboxyl moieties on the polyacrylic acid backbones. (See Kinoshita, Y., et. al., Biomaterials 14:209-215, 1993).
Generally, the resultant coating in the above-captioned situations is provided in the form of bioactive groups covalently coupled to biomaterial surfaces by means of short linear spacers. This approach works well with large molecular weight bioactive groups, such as collagen and fibronectin, where the use of short spacers is desired and the size of the bioactive group is quite large compared to that of the spacer itself.
The approaches described above, however, with the possible exception of Kinoshita et al., are not optimal for coating small molecular weight bioactive groups. Kinoshita does appear to coat small molecular weight molecules, although it describes a laborious multistep process that can detrimentally affect both yield and reproducibility.
Small molecular weight bioactive groups are typically provided in the form of either small regions derived from much larger molecules (e.g., cell attachment peptides derived from fibronectin) or as small molecules that normally diffuse freely to produce their effects (e.g., antibiotics or growth factors). It appears that short spacers can unduly limit the freedom of movement of such small bioactive groups, and in turn, impair their activity when immobilized. What are clearly needed are methods and compositions for providing improved concentrations of bioactive groups, and particularly small molecular weight groups, to a biomaterial surface in a manner that permits improved freedom of movement of the bioactive groups.